Geophysical evidence for an enriched molten silicate layer above Mars’s core

The detection of deep reflected S waves on Mars inferred a core size of 1,830 ± 40 km (ref. 1), requiring light-element contents that are incompatible with experimental petrological constraints. This estimate assumes a compositionally homogeneous Martian mantle, at odds with recent measurements of anomalously slow propagating P waves diffracted along the core–mantle boundary2. An alternative hypothesis is that Mars’s mantle is heterogeneous as a consequence of an early magma ocean that solidified to form a basal layer enriched in iron and heat-producing elements. Such enrichment results in the formation of a molten silicate layer above the core, overlain by a partially molten layer3. Here we show that this structure is compatible with all geophysical data, notably (1) deep reflected and diffracted mantle seismic phases, (2) weak shear attenuation at seismic frequency and (3) Mars’s dissipative nature at Phobos tides. The core size in this scenario is 1,650 ± 20 km, implying a density of 6.5 g cm−3, 5–8% larger than previous seismic estimates, and can be explained by fewer, and less abundant, alloying light elements than previously required, in amounts compatible with experimental and cosmochemical constraints. Finally, the layered mantle structure requires external sources to generate the magnetic signatures recorded in Mars’s crust.

Expectedly, mantle velocities are slightly higher and allow for a shallow low-velocity zone in the new model.Unexpectedly, core velocities are presented as identical for the bigger, cooler core with a higher percentage of Fe in the new model and the hotter, smaller, less iron-rich core of the old model*.Moreover, the new model contains less volatiles than the old model, which also tends to lower velocities.* Added note: Fig 2 shows that a P-velocity discontinuity is presented inside the liquid core, 50 km below the boundary between the BML and the liquid core, which is not discussed.This boundary is plotted about 50 km higher (~1700 km) than where the text and other figures suggest it should be (~1640 km?).
The new model is based on comprehensive multi-disciplinary constraints, but also fits seismic data (from InSight) better: 1.The new model increases the time between PP and SS arrivals by 10 s by allowing both to travel well below the thinner lithosphere, while in the old model the SS wave traveled within the highvelocity lithosphere, arriving earlier compared to the PP wave.2. The new model reduces the time between the Pdiff and the PP arrivals by 40 s by allowing the boundary-grazing Pdiff wave to refract into the low-velocity BML before returning to the mantle and surface and arriving later**.3. The new model reduced the time between PP and SKS by 10 s by allowing the SKS wave to travel through a slight faster mantle overall, while the PP wave's arrival is experiences a smaller net effect from a thinner lithosphere and cooler mantle.***** Added note: The CMB-grazing bottom part of the Pdiff wave in Fig 1l is weird.Ray paths that impinge on such a boundary typically refract into the bottom layer while bending away from the normal.So, I expected the Pdiff wave to become a PKP wave, rather than surface where it was observed.*** Added note: There is no discussion of this in the paper, though there ought to be, given how important this constraint is.It is also the only constraint on core velocities: Can it be used to better estimate core velocities?
The paper is in pretty great, but not perfect shape.I have some comments about how the presentation of the work can be improved before publication, which I recommend.
A. Equation 2 specifically includes dependence on the radius while equation 1 does not.Yet, eqn 1's dependence on radius is a critical ingredient in the discussion on page 6 on increasing viscosity with depth in the BML model.B. Page 9 refers to [attenuation in] the solid mantle, while earlier pages refer to the convecting mantle.Do these two terms describe the same part of the mantle?C. Page 10: "A consequence of a BML on Mars is that its early magnetic field recorded in the crust [58-60] would result from one or several external sources, such as impacts [61], or elliptical core instabilities excited by early satellites orbiting the planet in retrograde fashion [62].This is because the thermal buffering effect of the BML and its heating due to the enrichment in HPEs results in core heating rather than cooling [3], therefore reducing the duration of a thermal dynamo in Mars' core."This paragraph is underdeveloped and comes across as an afterthought.It assumes that readers will know what a thermal dynamo is, when it can have been active, and why cooling is required for a planetary dynamo.D. The discussion on page 4-5 and Fig 1 of mantle evolution with and without a BML is essential but hard to follow due to its qualitative (rather than quantitative) nature (relatively hot, relatively cold, rapid, increases, decreases, depletes, reduces, hotter, shallow, low viscosities).It also vaguely, and likely unintentionally implies that the present-day layering reflects initial layering and that, for example, an initially homogeneous mantle does not evolve into a crust-lithospheremantle-BML assemblage.E. page 6: "However, the differences in V * imply that the mantle becomes considerably more viscous with depth for BML models because the pressure and temperature gradients in the convecting mantle are similar for the output models of both inversion sets.Indeed, neglecting the relatively small temperature effect compared to that of pressure, and assuming a pressure increase of ∆P ∼ 15 GPa throughout the convecting mantle, Eq. 1 implies a lowermost to uppermost solid mantle viscosity ratio, R ∼ exp [∆P V * /(R T )], leading to R ∼= 50 for non-BML models, and R ∼= 5000 for BML models."This is all a bit confusing.a. E* being smaller is understandable on account on the lower present-day T for SML models.But why is V* so much larger in the BML models?b. "mantle become more viscous with depth": is this in the part of the mantle that is ABOVE the BML? c.If the p and T gradients are similar in BML and homogenous-mantle models, why does eta increase with depth only in the BML model?d. "small temperature effect compared to that of pressure" Why, where, and when is this happening?Is this related to gradients in E* and V* with depth?Is this happening in the mantle between the lithosphere and BML?Is this the case at present or initially?e. "assuming a pressure increase of ∆P ∼ 15 GPa throughout the convecting mantle": An increase from where to where?top to bottom?"throughout" might not be the right word to use.F. I have three concerns with Figure 1: 1) The core for the BML model (right) is 700 K hotter than the core in the homogeneous model (left), yet the P velocities in the core seem to be the same, as if the bulk modulus and density are not sensitive to temperature in liquid iron under pressure?
2) The crust for the BML model is 60 km thick and the crust for the homogenous model is 80 km thick.Neither thickness agrees well with those inferred by InSight (30-70 km).In the homogeneous model that is remedied by the bottom part of the crust having seismic velocities that are virtually identical (or very close to) mantle velocities, while still having the same (low) density as the upper crust.This requires a lot of explanation I have not seen in this paper or earlier InSight papers on the crust.3) "TBL" is used in the figure but not defined.
G. "In addition to seismic arrival times, the detection of distant events of small amplitude by InSight's seismic experiment suggests that seismic attenuation is small, with effective shear quality factors in excess of 1000 [9]."Do you mean "In addition to seismic arrival times, the detection of distant events of small MAGNITUDE by InSight's seismic experiment suggests that seismic attenuation is small, with effective shear quality factors in excess of 1000 [9]."H. "These lines of evidence suggesting the existence of a BML motivate the re-interpretation of available data in the frame of a deep layered Martian mantle."What is meant with "deep layered mantle"?Do you mean "These lines of evidence suggesting the existence of a previously unrecognized BML motivate the re-interpretation of available data used to constrain the size of Mars' core."I. Page 5: "Therefore, from a seismological point of view, the BML acts essentially as an extension of the core for deep reflected S-waves" This presumes that the core itself is liquid, as suggested in previous publications, but this is not been stated in the current manuscript until the bottom of page 9.It probably needs to be said earlier. 4 shows the 100 best models.This inconsistency has the appearance or arbitrariness, which might imply there to be no quantitative criteria for model selection.

J. Fig 3 shows the 1000 best models and Fig
K. Supplement: The color scales in Figures S1 and S2 are not perceptually uniform, making the images hard to interpret, even for someone who is not colorblind.Please use a perceptually uniform scale, like viridis.L. Because the seismograms from InSight have a low Signal to Noise Ratio, it is not straightforward to quality-control the arrival times used in the paper within a reasonable amount of time.In addition, most of the arrival times have already been published, hence peer-reviewed, elsewhere.
M. Supplement: The authors state to have used equations from Aki & Richards's chapter 5 to calculate SV reflection coefficients on the boundary between lower mantle and BML.Which equation specifically was or were used?My version seems to be missing the equations for solidliquid boundaries.And did they use one sharp boundary or cumulative coefficients for the transition layer (partially molten) between lower mantle and the liquid BML?I have been asked to review a paper concerning the evidence and implications of a molten silicate layer at the core-mantle-boundary (CMB) of Mars.The paper presents an internal structure model of Mars that has recently been sharpened by the occurrence of impact-driven quakes that generated waves grazing the CMB (S1000a and S0976a).The paper presents conclusions concerning the thickness of the molten layer, ~150-170 km.The major consequence is that by reducing the liquid core radius, its density increases by ~8%, thus reducing the concentration of light elements (S, C, H, O) required to match the density of molten iron.Such a reduction makes the concentration of light elements required more plausible since it is more consistent with cosmochemical models.
A foundational ingredient of this modelling is the ab initio density and bulk modulus of liquid iron and iron alloys at Mars' core pressures.In this pressure range (~19 < P < ~37 GPa), the equation of state of liquid or solid iron is not well reproduced by any exchange-correlation functional I am aware of.The agreement improves above 100 GPa at room temperature.Without such reliable essential information, a solid solution model for FeS-Fe, FeO-Fe, Fe3C-Fe, and for fcc-FeH are developed.The parameters of such models are given in the paper, as well as the density and compressional velocities of these alloys vs. pressure.However, it isn't easy to understand how the core composition modelling was done.One sentence was offered without further elaboration: "Oxygen concentration depends on the concentration of sulfur in the core, the amount of FeO in the mantle, and the conditions of core formation Gendre et al. [2022]..." This sentence is packed with unjustified assumptions.Another sentence: "We, therefore, carried out multi-stage core formation models Badro et al. [2015] scanning all the possible values of input parameters (S content, FeO concentration, magma ocean geotherm, CMB pressure) and determined a range of oxygen concentrations as a function of sulfur concentration."This procedure also needs to be better explained.The authors should give a lot more details on how the compositional models were developed.The uncertainty in the alloys' EoSs is passed on to the core composition, which is modelled to fit the new Martian core model.
The bottom line is: the EoS of iron and light element alloys produced in this paper supports the desired conclusion, i.e., the consistency with cosmochemical constraints.This paper explores an enriched partially molten silicate later at the base of Mars' mantle.The paper presents a nice parallel approach for the two contrasting hypotheses: an inversion with and without the BML is pursued.In general, I find the paper interesting, but the seismic results seem a bit uncertain.Here are some comments/questions:

Reviewer Reports on the First Revision:
Referee #1:

Review of Evidence and implications of an enriched molten silicate layer above Mars' core
Based on the rebuttal and scanning the revised manuscript and supplementary information, I am happy with the extensive improvements the authors have made to an already great study.
However, I have one lingering question concerning my original question: E. page 6: "However, the differences in V* imply that the mantle becomes considerably more viscous with depth for BML models because the pressure and temperature gradients in the convecting mantle are similar for the output models of both inversion sets.Indeed, neglecting the relatively small temperature effect compared to that of pressure, and assuming a pressure increase of ∆P ∼ 15 GPa throughout the convecting mantle, Eq. 1 implies a lowermost to uppermost solid mantle viscosity ratio, R ∼ exp [∆P V*/(R T )], leading to R ∼= 50 for non BML models, and R ∼= 5000 for BML models."This is all a bit confusing.a. E* being smaller is understandable on account on the lower present-day T for SML models.But why is V* so much larger in the BML models?
The authors' answer included a re-iteration of how this V* (activation volume) is the result of their data-driven inference process.However, my lingering question is whether the authors could provide a physical justification/intuition for the large difference in V* between the BML and non-BML models.
Other than that, I am excited to see this multidisciplinary paper in print.The description of the core properties modeling has improved and it is clearer now what has been done.Because there is a great deal of extrapolation from measurement conditions to core conditions, the uncertainty on the extrapolated property might not be much smaller than the error derived from "adjusted" ab initio calculations.This approach does not seem necessarily superior to "adjusted" ab initio results.Nevertheless, there is no alternative left.I trust the materials modeling component is reasonable.

Review of Evidence and implications of an enriched molten silicate layer above Mars' core
Based on the rebuttal and scanning the revised manuscript and supplementary information, I am happy with the extensive improvements the authors have made to an already great study.
However, I have one lingering question concerning my original question: E. page 6: "However, the differences in V* imply that the mantle becomes considerably more viscous with depth for BML models because the pressure and temperature gradients in the convecting mantle are similar for the output models of both inversion sets.Indeed, neglecting the relatively small temperature effect compared to that of pressure, and assuming a pressure increase of ∆P ∼ 15 GPa throughout the convecting mantle, Eq. 1 implies a lowermost to uppermost solid mantle viscosity ratio, R ∼ exp [∆P V*/(R T )], leading to R ∼= 50 for non BML models, and R ∼= 5000 for BML models." This is all a bit confusing.a. E* being smaller is understandable on account on the lower present-day T for SML models.But why is V* so much larger in the BML models?
The authors' answer included a re-iteration of how this V* (activation volume) is the result of their data-driven inference process.However, my lingering question is whether the authors could provide a physical justification/intuition for the large difference in V* between the BML and non-BML models.
We are sorry for having missed the reviewer's point in the first round of review.The rheology of planetary mantles is not straightforward to predict or to explain because of the remaining tradeoffs between poorly constrained quantities that can affect the rheological behaviour of the mantle.For example, an intrinsically more sluggish mantle can be explained by a drier mantle (Karato & Wu, Science, 1993) but also by larger grain sizes, which are not explicitly modeled in our inversions.Therefore, explaining/justifying further the rheological differences between the two inversion outputs would be too speculative, because so far we cannot constrain the associated quantities that could explain these rheology (water content, grain size, major element content, etc.) or break the existing tradeoffs between these quantities on the rheology.We can only defer this quest to a future study.We added a text in the last version of the revised Methods Section to underline this point.
Other than that, I am excited to see this multidisciplinary paper in print.The description of the core properties modeling has improved and it is clearer now what has been done.Because there is a great deal of extrapolation from measurement conditions to core conditions, the uncertainty on the extrapolated property might not be much smaller than the error derived from "adjusted" ab initio calculations.This approach does not seem necessarily superior to "adjusted" ab initio results.Nevertheless, there is no alternative left.
We disagree with these statements.Our EoS for the core is based on an extensive set of experimental data acquired at conditions below, within, and above those prevailing inside the core of Mars (as mentioned in the supplement of our manuscript).Therefore, our EoS applied to Mars' core does not require extrapolation.
In particular, the non-ideal solution model for Fe-S (S likely being the most abundant light element in the core) is based on 89 different density measurements (conducted in the range 2-43 GPa) and 28 acoustic velocity measurements (conducted in the range: 0-52 GPa) for 23 different sulfur concentrations (5-37.5 wt.%).Densities and acoustic velocities predicted by our thermodynamic model at the experimental conditions are well within the uncertainties of the latter.Using Monte Carlo error propagation, the uncertainty is respectively less than 1% and 2% for density and acoustic velocity.
Our thermodynamic model accurately summarizes the relevant available experimental data, and can predict with a high confidence level the thermodynamic properties of the whole core at conditions (composition, pressure, temperature) falling within the range covered by the experiments.For this reason, our core model based on experiments is currently the only viable option to infer core composition.As argued in our previous rebuttal letter, ad hoc models build from a small number of "adjusted" (e.g., Huang et al.; 2023) ab initio calculations, are unreliable because they individually deviate significantly from experimental data.
Finally, the EoS used in our core model for the remaining liquid state end-members do not require pressure extrapolation either (Fe up to 350 GPa; FeO up to 100 GPa; Fe-C up to 50 GPa; Fe-H up to 152 GPa).

Author Rebuttals to Second Revision:
I trust the materials modeling component is reasonable.
We are happy that our changes made to the SI clarified our approach.We thank the reviewer for highlighting the potentially confusing way that the raypaths were presented in Fig. 1n.In this figure we chose to plot a single raypath despite the fact that the diffracting nature of the Pdiff means that energy is continuously radiated into the molten layer beneath and reflected back toward the base of the fully solid mantle, where it resumes its diffracting path.All these paths have identical travel times and the energy that travels along them constructively interferes to produce the prominent signal we analyze in the manuscript.Naturally, for a surface event, the inherent symmetry of the geometry implies that the ensemble of all these paths must be symmetric.However, only the single path whose "PcP" bounce is at the midpoint of the raypath will itself also be symmetric; on either side, it will be flanked by a distribution of other paths that will not be individually symmetric.The red path in Fig. 1 is one such non-symmetric path and is not hand-drawn.We showed a subset of other paths in the "Pdiff-PcP" ensemble in Fig. S4 and now point the reader to this figure for a better sense of the range of paths that are associated with the observed arrival.We therefore have left the path displayed in Fig. 1n but added text to the figure caption to explain the asymmetry. .

Fig S11 did not come through on the PDF I downloaded for review
This is hopefully fixed upon resubmission of the last version of our revised manuscript.
I feel the paper now adequately supports the hypothesis put forward.
Excellent news.Thank you.

*
Fig S11 did not come through on the PDF I downloaded for review

3 :
The revised manuscript is improved and addresses questions in my first review.Some minor comments: I don't understandFig 1(n): there should be reciprocity on either side of the "PcP" segment, but the P(diff) portions are different on either side of the paths in the raypath figure.The red path looks hand drawn.Is it?If yes, why not show the actual computed path?